Recovery heat treatment of highly strained components

ABSTRACT

A method of recovery heat treatment of a workpiece includes contacting at least a portion of the workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to a predetermined temperature range in a predetermined time period. The method may be implemented as a first step of a two-stage recovery heat treatment or annealing process, followed by a second step of heating the at least a portion of the workpiece to a final target temperature by another annealing process.

TECHNICAL FIELD

This disclosure relates generally to the metal forming field and, more particularly, to a method for recovery heat treatment of a workpiece to provide for enhanced formability and to reduce defect-related or strain-related splits during processes of bending, pre-forming, hydroforming, secondary forming, drawing, redrawing, restriking, stamping, flanging, etc. of the workpiece.

BACKGROUND

Hydroforming is a term applied to metal forming in which the metal is formed against a die by internal fluid pressure. This may be done with an internal fluid pressure with an applied axial load to a tube or in the case of sheet metal with a one-sided die in which the sheet metal is formed by a bladder/diaphragm. Tube hydroforming typically uses conventional, single action hydraulic presses with high ram forces.

In one example, hydroformed aluminum closed or “boxed” rails may be used to form the frame rails, A-pillar roof rails, etc. of a vehicle structure. This type of part may be made with structural (porthole) or seamless extruded tubes. Structural tubes have better wall and diameter dimensional tolerances and are more efficient to extrude but have lower formability for bending, pre-forming, and hydroforming processes. As a consequence, structural tubes can typically only be used to form less challenging component shapes in hydroforming processes. Seamless tubes can have significantly higher formability which has made them the preferred material for hydroforming parts having complex cross-sectional geometries, such as A-pillar roof rails and frame rails. However, seamless tubes are relatively expensive compared to structural tubes. This is because seamless tubes are less efficient to extrude relative to structural tubes due to low output one-out extrusion process and seamless press cycle time limitations. Seamless tubes also have wall and diameter dimensional tolerances that can be at least twice that of structural tubes.

When using a multiple stage approach to hydroforming various bending/pre-forming steps may occur prior to the final hydroforming of the part. During these initial deformation stages much of the material formability can be exhausted leaving little plastic strain capability for the hydroforming process itself. For example, during hydroforming processes used in manufacture of workpieces such as vehicle boxed frame rails, splits can occur in highly strained areas of the workpieces. Such splits are potentially linked to other aggravating factors, one non-limiting example being galling occurring during bending and/or pre-forming processes, and result in unusable or inadequate workpieces and attendant cost of materials and labor in fabricating new workpieces. While to some extent controllable, such defect-related splits are difficult to eliminate completely.

One way to mitigate the influence of these local defects and improve formability is to apply a recovery heat treatment between stages of the hydroforming process. However, conventional recovery heat treatment processes such as induction annealing, while effective, have limited applicability to certain vehicle components such as boxed frame rails due to their size and complex geometries. Therefore, to solve this and other problems the present disclosure is directed to recovery heat treatment processes to alleviate defect-related splits such as are encountered during workpiece bending, pre-forming, and/or hydroforming processes, and to improve overall formability of workpieces. Advantageously, the described process allows use of less expensive structural tube extrusions for fabricating workpieces such as frame rails, roof rails, A-pillar rails, and the like.

SUMMARY

In accordance with the purposes and benefits described herein, in one aspect a method of recovery heat treatment or annealing is provided, comprising contacting at least a portion of a workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to a predetermined temperature range in a predetermined time period. The workpiece may comprise a metal or alloy for example an aluminum alloy such as a 6xxx or a 5xxx aluminum alloy. Such aluminum alloys include without intending any limitation an AA6082 aluminum alloy, an AA6061 aluminum alloy, an AA6063 aluminum alloy, an AA6111 aluminum alloy, an AA6022 aluminum alloy, an AA6016 aluminum alloy, an AA5754 aluminum alloy, or an AA5182 aluminum alloy. The workpiece may be a bent and/or pre-formed vehicle boxed frame rail, front rail, or roof rail. Alternatively, the workpiece may be a pre-formed sheet metal body inner or outer panel, or structural component.

In embodiments, the method includes a step of contacting the at least a portion of the workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to a predetermined annealing temperature range in a predetermined time period. In embodiments, the method may include contacting only a bent portion of the workpiece with the fluid. In embodiments, the fluid may be a liquid, and the workpiece may be fully or partially immersed therein.

In another aspect, a method of recovery heat treatment or annealing of a workpiece is described, comprising a first step of heating at least a portion of the workpiece to a first, intermediate temperature range in a predetermined time period. In a next step, the at least a portion of the workpiece is heated to a second, annealing temperature range in a predetermined time period. The step of heating to a first, intermediate temperature range of the at least a portion of the workpiece may be accomplished with a suitable heat source. In embodiments, the at least a portion of the workpiece may be heated to the intermediate temperature by a heated fluid substantially as summarized above. In other embodiments, the heat source may be one or more heating elements. In embodiments, the step of heating the at least a portion of the workpiece to a final target temperature may be accomplished by an induction or other suitable annealing process.

In the following description, there are shown and described embodiments of the disclosed recovery heat treatment processes. As it should be realized, the processes are capable of other, different embodiments and their several details are capable of modification in various, obvious aspects all without departing from the devices and methods as set forth and described in the following claims. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated herein and forming a part of the specification, illustrate several aspects of the presently described methods and together with the description serve to explain certain principles thereof. In the drawings:

FIG. 1 illustrates a workpiece received in a channel coil for purposes of a prior art induction annealing process;

FIG. 2A depicts in flow chart form a representative method for hydroforming a rail workpiece including a recovery heat treatment process according to the present disclosure;

FIG. 2B depicts in flow chart form a representative method for forming a planar workpiece including a recovery heat treatment process according to the present disclosure;

FIG. 3 depicts the recovery heat treatment process of FIG. 2;

FIG. 4 depicts in flow chart form a representative method for hydroforming a rail workpiece including a two-stage recovery heat treatment process according to the present disclosure; and

FIG. 5 depicts a heating element arrangement suitable for accomplishing the two-stage recovery heat treatment process of FIG. 4.

Reference will now be made in detail to embodiments of the disclosed methods, examples of which are illustrated in the accompanying drawing figures.

DETAILED DESCRIPTION

Preliminarily, the description that follows described recovery heat treatment processes primarily in the context of a hydroforming process for a pre-strained metal rail workpiece. However, it will readily be appreciated that the described recovery heat treatment processes apply equally to pre-strained planar workpieces such as sheet metal. In turn, the described recovery heat treatment processes apply equally to other metal-forming processes, including without intending any limitation processes of metal bending, pre-forming, secondary forming, drawing, redrawing, re-striking, stamping, flanging, and others.

One known process for recovery heat treatment for pre-strained metal workpieces is induction annealing, which uses alternating current flowing through an induction coil to create an electromagnetic alternating field which, in turn, induce Eddy current to heat a workpiece. The process requires a water-cooled copper coil (primary transformer) that follows the shape of the workpiece (secondary transformer) and accommodates the variables of the process (frequency, voltage, and others), coil, and workpiece. This process is illustrated in FIG. 1 which depicts a water-cooled induction coil 100 for heat treating a tubular workpiece 102. The voltage level depends on the number of electrical turns in the primary transformer. Heating of the workpiece is accomplished by the induced current (arrows I) causing a Joule heating effect. Heating efficiency and uniformity is achieved through coil design and contouring relative to the workpiece 102 design/shape. Overall, induction annealing is an efficient and effective process for annealing discrete portions of a workpiece 102.

Therefore, induction annealing was considered as a potential method for recovery heat treatment to alleviate occurrence of splits during workpiece pre-forming, bending, hydroforming, etc. However, induction annealing was found to be unsuitable for certain workpieces due to their size and/or conformation and/or location of target areas to be annealed. For example, for pre-formed and/or bent tubing used to form vehicle boxed frame rails and other such components, it was found that cycle time and temperature requirements rendered induction annealing inconvenient and unfeasible for incorporation into the manufacturing process.

In more detail, in considering tube hydroforming/annealing processes for aluminum or other metal or alloy tubing intended for fabrication into vehicle boxed frame rails and other such components it was found that no suitable level of energy/heat flux induced by an induction coil could achieve a desired average target temperature of 140±10° C. within a desired 21 second cycle time. Higher levels of flux were able to achieve this average target temperature within the 21 second cycle time, but produced unacceptable temperature gradients in excess of 300° C. On the other hand, using lower levels of flux to achieve the desired average target temperature of 140±10° C. required cycle times of at least 90 seconds, which significantly exceeded the desired 21 second production cycle time target.

Therefore, alternative recovery heat treatment methods were considered. At a high level, the present disclosure describes a recovery heat treatment method for raising a workpiece or workpiece portion to a desired target annealing temperature. The described recovery heat treatment method may also be provided as a first step in a multiple-step annealing process wherein a workpiece is heated to a desired intermediate temperature using the presently described recovery heat treatment process, and then in a subsequent second step is heated to a final target temperature using a second heating process. In embodiments, the second heating step may comprise the induction annealing process described in U.S. Published Patent Appl. No. 2015/0315666 to Harrison et al. published on Nov. 5, 2015 and entitled “Induction Annealing as a Method for Expanded Hydroformed Tube Formability,” the entirety of the disclosure of which is incorporated herein by reference. However, use of other known annealing processes for the second heating step is contemplated.

The presently described processes may be incorporated as a portion of a process of hydroforming the workpiece 102, for example the hydroforming processes described in U.S. Published Patent Appl. No. 2015/0315666, comprising various steps performed in varying order of bending, annealing, pre-forming, hydroforming, trimming, and aging the workpiece. This representative hydroforming process 200 for a rail is illustrated in FIG. 2A. The process illustrated in FIG. 2A comprises steps of bending a workpiece (not shown) into a first preliminary shape (see step 202). This is followed by recovery heat treatment process applied to the workpiece (see step 204). The recovery heat treatment process 204 locally heats the workpiece in order to alleviate excessive strain hardening within the workpiece thereby allowing for increased formability during later stages of the hydroforming method.

This is then followed by pre-forming the workpiece into a second preliminary shape (step 206). Next, the workpiece is hydroformed to a desired final shape (note step 208). Subsequent to hydroforming, the workpiece may optionally be trimmed to a desired length (step 210. Next, the workpiece may be subjected to a heat treatment in order to impart desired strength properties to the workpiece (note step 212). In one exemplary embodiment the heat treatment can be a T6 treatment at 180° C. for six hours in order to induce or impart average yield strength of typically 290 MPa to the workpiece. In alternative embodiments the heat treatment may be completed at temperatures between 160-200° C. for 4 to 10 hours. In still other alternatives, natural aging without heat treatment may be implemented to allow the workpiece to harden to the desired average yield strength.

Of course, alternative step orders are contemplated as described in U.S. Published Patent Appl. No. 2015/0315666. For example, the steps of bending and pre-forming may precede the step of recovery heat treatment. Alternatively, each step of bending and pre-forming may be followed by a separate step of recovery heat treatment. All such alternative step orders are contemplated herein.

A similar process 214 for forming a planar workpiece such as sheet metal is illustrated in FIG. 2B, showing steps of blanking (step 216), primary forming (step 218), recovery heat treatment according to the present disclosure (step 204), secondary forming (step 220), and optionally trimming (step 222). Again, alternative step orders are contemplated.

For purposes of illustration, processes 204 of recovery heat treatment/annealing a vehicle boxed front frame rail formed of a bent and/or preformed extruded aluminum alloy are described herein. In the described embodiments, the aluminum alloy is a 6xxx aluminum alloy or a 5xxx aluminum alloy including without intending any limitation an AA6082 aluminum alloy, an AA6061 aluminum alloy, an AA6063 aluminum alloy, an AA6111 aluminum alloy, an AA6022 aluminum alloy, an AA6016 aluminum alloy, an AA5754 aluminum alloy, or an AA5182 aluminum alloy. However, the skilled artisan will appreciate that the described processes equally apply to other workpieces and other workpiece compositions, for example sheet metal, steel and other alloys, etc., and so the descriptions should not be taken as limiting.

As summarized above, in an embodiment the step 204 of recovery heat treatment of the workpiece according to the present disclosure comprises contacting at least a portion of the workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to a predetermined temperature range in a predetermined time period providing a desired annealing effect in a time frame conducive to conventional manufacturing processes. As will be appreciated by the skilled artisan, the term “fluid” describes substances that continually deform (flow) under an applied shear stress, and encompasses various phases of matter including liquids, gases, plasmas, and to a degree plastic solids. In an alternative embodiment, the step 204 of recovery heat treatment of the workpiece comprises contacting at least a portion of the workpiece with a fluid comprising a solid, all heated to a temperature sufficient to heat the at least a portion of the workpiece to the predetermined annealing temperature range in the predetermined time period.

This step 204 includes contacting at least a portion of the workpiece with an annealing substance heated to a suitable annealing temperature range, i.e. heated to a temperature sufficient to heat the at least a portion of the workpiece to a suitable annealing temperature. As will be appreciated, specific annealing temperature ranges will vary in accordance with physical, mechanical, and chemical properties of the workpiece, of the annealing substance, and with target annealing cycle times for a particular production setting. For example the specific annealing temperatures required to anneal a particular aluminum alloy will vary in accordance with the alloy properties. As will be appreciated, the skilled artisan is readily able to ascertain specific annealing temperature ranges for particular metals and metal alloys. For purposes of example only, particular combinations of annealing temperature and alloy are set forth in U.S. Published Patent Appl. No 2008/0105023 to Golovashchenko et al., published on May 8, 2008 and incorporated herein by reference.

In an embodiment, this step 204 includes contacting the workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to a temperature of from about 130° C. to about 150° C. in 20 to 30 seconds. Some or all of the workpiece may be contacted with the fluid. For example, as shown in FIG. 3 only a bent portion of the workpiece, being the portion with a high amount of accumulated plastic deformation and, thereby, most susceptible to defect related splits, may be contacted by the fluid.

In embodiments, the fluid may be a liquid or a gas. In embodiments comprising contacting the workpiece with a liquid, the liquid may be selected from the group consisting of an oil, a lubrication oil, a quenching oil, a heating oil, a synthetic oil, a semi-synthetic oil, a mineral oil, water, a water-oil emulsion, a polymer water, oil based solutions and emulsions, a molten salt such as sodium and potassium nitrates, nitrites, and chlorides, alkali salts, and combinations.

In other embodiments, the annealing step may be accomplished by contacting at least a portion of the workpiece with a solid, such as in a fluidized bed device including a fluid comprising dry, finely divided solid particles. Such devices are known in the art, being devices wherein fluid/solid mixtures are held under conditions causing the mixtures to behave as a fluid.

It will be appreciated that the particular recovery heat treatment substance (fluid, liquid, gas, liquid/solid mixture) will be selected according to the physical, chemical, mechanical, and other properties of the workpiece and the target annealing temperatures and/or cycle times to be implemented. Specifically, the substance will be selected whereby the target annealing temperature to be reached will be above the substance melting temperature but below the substance flashpoint temperature. In turn, the evaporation rate at the target annealing temperature should be considered, as well as compatibility with down-stream processing equipment/processes. In general, water-based substances are suitable for lower target annealing temperature ranges, oil-based substances are suitable for intermediate target annealing temperature ranges, and molten salt-based substances are suitable for higher target annealing temperature ranges, although the ranges may overlap. Selection of a particular annealing substance is well within the abilities of the skilled artisan.

As shown in FIG. 3, some or all of the workpiece 102 may be contacted with a liquid 300, for example by immersing at least a portion of the workpiece in a container 302 holding the liquid. In the depicted embodiment, a “two-sided” exposure, i.e. contact of the heated liquid 300 with both inner and outer surfaces of the portion of the workpiece subjected to recovery heat treatment, is provided. However, it will be appreciated that partial submersion resulting in contact of only one of an outer surface or an inner surface of the bent portion 304 of the workpiece with the liquid 300, i.e. a “one-sided” exposure, is also contemplated.

Advantageously, the process illustrated in FIG. 3 allows annealing or recovery heat treatment of component that are particularly large and/or that define a complex exterior geometry at a higher heating rate/shorter cycle time than would be possible using other annealing processes. For example, the presently described process allows providing uniform heating of workpieces 102 having a much larger size and more complex geometries than is possible by induction annealing, due to limitations imposed by the configuration of the induction coil apparatus 100 (see FIG. 1). In turn, the simplicity of the illustrated process results in significant cost savings during the recovery heat treatment and subsequent hydroforming processes.

In another embodiment, consideration was given to situations where a desired target temperature range required for annealing a workpiece 102 or workpiece portion might exceed a boiling point of a desired liquid 300, or situations when for safety reasons a maximum liquid 300 temperature cap would be required that was lower than the desired target temperature range for annealing a workpiece 102. In such situations, a two-stage annealing process was contemplated.

With reference to FIG. 4, a two-stage recovery heat treatment process for a rail workpiece is illustrated as a portion of a hydroforming process 400. While not specifically depicted, it will be readily appreciated that the described two-stage recovery heat treatment process is easily incorporated into processes for forming a planar workpiece such as sheet metal as is illustrated in FIG. 2B. Substantially as shown in FIG. 2, the process 400 includes steps of bending (step 402), recovery heat treatment (step 404), pre-forming (step 406), hydroforming (step 408), optionally trimming (step 410), and optionally aging (step 412). Again, alternative step orders are contemplated. For example, the steps of bending and pre-forming may precede the step of recovery heat treatment. Alternatively, each step of bending and pre-forming may be followed by a separate step of recovery heat treatment. All such alternative step orders are contemplated herein.

The two-stage recovery heat treatment process includes a first step 404 a of heating at least a portion of the workpiece (not shown) to a first, intermediate temperature by immersing some or all of the workpiece 102 in a suitably heated fluid or solid, substantially as described above and as shown in FIG. 3. The specific intermediate temperature range and time period may be fixed, or may vary according to physical, chemical, and mechanical properties of the workpiece 102 as described above.

Alternatively, the workpiece 102 may be preheated to bring the workpiece to the desired intermediate temperature range using alternative heat sources. In one embodiment the heat source is a suitable heating element. As shown in FIG. 5, in one possible embodiment a conveyer 500 passes one or more workpieces 102 past one or more heating elements 502. The workpieces 102 may be arranged such that a specific portion of the workpieces (bend 504) is closest to the heating elements 502, which in the depicted embodiment are installed substantially above where the workpieces will pass, and so will receive the most heating.

In the depicted embodiment, heating elements 502 are infrared heaters (also referred to as heat lamps), which as is known transfer heat by radiation (emitting energy in the infrared wavelengths). However, alternative heating element types are contemplated, including without intending any limitation heat guns, pressurized blowers for delivering a heated gas over a surface, or exposure of the workpieces 102 to a low temperature convection or radiation oven. This could occur by placing the workpieces 102 in the described oven, or alternatively by passing a conveyer such as is shown in FIG. 5 through such ovens.

Once the workpiece or desired portion of the workpiece has been heated to the first, intermediate temperature range, the workpiece may then be heated to a second, annealing temperature range (step 404 b) to complete the two-stage recovery heat treatment process 404. In an embodiment, the workpiece may be heated to the second, annealing temperature range by an induction annealing process substantially as summarized above (see FIG. 1) and more fully described in U.S. Published Patent Appl. No. 2015/0315666. However, other suitable annealing processes are also contemplated.

In turn, the skilled artisan will appreciate that particular temperatures needed to heat all or a portion of a workpiece to a predetermined annealing temperature or temperature range (FIGS. 2A, 2B, 3), or to a predetermined intermediate temperature (FIG. 4) within a predetermined time frame according to the processes set forth above can readily be calculated in accordance with the known physical properties of the heating fluid/heating elements being used and of the workpiece. In an embodiment, the fluid or solid used to heat the workpiece or workpiece portion is heated to the predetermined annealing temperature or temperature range, or to the predetermined intermediate temperature or temperature range.

In summary, numerous benefits result from the described methods of recovery heat treatment 204, 404 of a workpiece 102 as disclosed herein. The methods support high volume automotive manufacturing. Any contemplated process of metal forming benefits from the method, including forming of both structural and seamless tubes. In fact, structural tubes may now be readily used in the production of difficult-to-form boxed frame rails, A-pillar roof rails, and others. Thus, the method allows for the use of a higher tolerance and more manufacturing efficient material for hydroforming roof rails.

Advantageously, the described methods are simple, efficient, and economical, and can be performed in a manner restricted to a specific region of interest to be annealed in a workpiece 102, for example a bend in a boxed frame rail, where plastic strain capability has been reduced by bending and/or pre-forming steps or stages during the production process. Heating of the workpiece 102 can be easily localized to the to-be-annealed region, and therefore there is no specialized equipment required for material handling of the workpiece in the unheated regions.

The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled. 

What is claimed:
 1. A method of recovery heat treatment, comprising contacting at least a portion of a workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to a predetermined annealing temperature range in a predetermined time period.
 2. The method of claim 1, wherein the workpiece comprises an aluminum alloy.
 3. The method of claim 2, wherein the aluminum alloy is selected from the group consisting of an AA6082 aluminum alloy, an AA6061 aluminum alloy, an AA6063 aluminum alloy, an AA6111 aluminum alloy, an AA6022 aluminum alloy, an AA6016 aluminum alloy, an AA5754 aluminum alloy, or an AA5182 aluminum alloy.
 4. The method of claim 1, wherein the workpiece is a bent and/or pre-formed vehicle frame rail or a formed sheet metal workpiece.
 5. The method of claim 1, including contacting only a formed portion of the workpiece with the fluid.
 6. The method of claim 1, wherein the fluid is a liquid.
 7. The method of claim 1, wherein the fluid comprises a finely divided particulate solid.
 8. The method of claim 6, wherein the liquid is selected from the group consisting of an oil, a lubrication oil, a quenching oil, a heating oil, a synthetic oil, a semi-synthetic oil, a mineral oil, water, a water-oil emulsion, a polymer water, oil based solutions and emulsions, a molten salt, an alkali salt, and combinations thereof.
 9. A method of recovery heat treatment of a workpiece, comprising: heating at least a portion of the workpiece to a first, intermediate temperature range in a predetermined time period; and heating the at least a portion of the workpiece to a second, annealing temperature range in a predetermined time period.
 10. The method of claim 9, wherein the workpiece comprises an aluminum alloy.
 11. The method of claim 10, wherein the aluminum alloy is selected from the group consisting of an AA6082 aluminum alloy, an AA6061 aluminum alloy, an AA6063 aluminum alloy, an AA6111 aluminum alloy, an AA6022 aluminum alloy, an AA6016 aluminum alloy, an AA5754 aluminum alloy, or an AA5182 aluminum alloy
 12. The method of claim 9, wherein the workpiece is a bent and/or pre-formed vehicle frame rail or a formed sheet metal workpiece.
 13. The method of claim 9, including contacting the at least a portion of the workpiece with a fluid heated to a temperature sufficient to heat the at least a portion of the workpiece to the first, intermediate temperature range.
 14. The method of claim 13, including contacting only a formed portion of the workpiece with the fluid.
 15. The method of claim 13, wherein the fluid is a liquid.
 16. The method of claim 13, wherein the fluid comprises a finely divided particulate solid.
 17. The method of claim 15, wherein the liquid is selected from the group consisting of an oil, a lubrication oil, a quenching oil, a heating oil, a synthetic oil, a semi-synthetic oil, a mineral oil, water, a water-oil emulsion, a polymer water, oil based solutions and emulsions, a molten salt, an alkali salt, and combinations thereof.
 18. The method of claim 9, including heating the at least a portion of the workpiece to the first, intermediate temperature range by one or more heating elements.
 19. The method of claim 18, wherein the one or more heating elements are infrared heating elements.
 20. The method of claim 19, including heating the at least a portion of the workpiece to the second, annealing temperature range by an induction annealing process. 